What do I do in the fire realm?

First, take a look at what the tree is doing.

It's touching the crystals.

But it's not the only thing on the screen that can touch the crystals.

Have you tried clicking on the sun?

When you do, it will shoot bolts of lightning at the crystals as well.

Click on the sun a few times, and you should notice that the sun will do
something else as well.

As long as your timing is right.

Whenever the tree and the lightning hit the crystals at the same time, a note
will play.

The lightning shoots out at all the crystals in a particular order.

You want the tree to touch the crystals in the same order and with the same
timing.

You can also change how the tree moves.

Have you tried clicking on the crystals yourself?

When you click on a crystal, the branch that touches the crystal will stop
moving for a bit.

In fact, the tree will freeze for as long as you keep your mouse button held
down.

A lot of this puzzle is just in fine tuning your timing, but the basic goal
is to "reprogram" the tree so that its branches move in sync with the
lightning, once you touch the sun.

One thing I found helped was to program the branches one at a time, starting
with the second one the lightning touched.  Just a bit after a branch touches
a crystal (i.e., the crystal just starts glowing), click on that crystal. 
Then, wait until a branch is touching the very _bottom_ of the preceeding
crystal in the pattern, and let go of your mouse button.